In general the present invention relates to exploration or production platforms for petroleum or mining.
One type of exploration or production platform is the semi-submersible. A semi-submersible has a deck supported by a plurality of columns that in turn attach to pontoons. The pontoons are totally submerged in the sea and the columns extend upwardly from the pontoons through the water surface and above it to the deck; the deck being spaced above the sea surface. The pontoons and columns below the water surface provide buoyancy. The deck provides the work area. Semi-submersibles are large structures. A typical deck may bee on the order of 90 meters across.
Swell or wave induced motion on a semi-submersible can be serious. (In general, swell is a wave having a narrow period spectrum. A storm sea, in contrast, is the resultant of individual waves of different periods and has a broad spectrum. In this specification, wave will be used to embrace both unless the content requires delineation.)
Heave is vertical motion and is one of the motions induced by waves.
Dynamic wave forces that produce a heave decrease with depth. Surfaces of constant pressure, isobars, in waves are farther apart under wave crests and closer together under wave troughs. The effect of waves on isobars is greatest close to the wave surface. At greater and greater depths, the isobars flatten out until wave influence disappears entirely, and the isobars become equally spaced horizontal surfaces.
Heave occurs because of changes in the vertical forces acting on a platform; a change in the shape of the isobars. In quiet water without waves, a platform experiences only its gravity force acting downward and an equal and opposite buoyant force acting upward. Accordingly, the platform experiences no vertical motion. When waves disturb the water, additional, dynamic forces act on the platform. In the case of a column that passes through the water surface, the wave force acting on it in heave is in phase with wave motion: the wave force is a positive maximum at a wave crest and a negative minimum at a wave trough, with positive and negative being with respect to buoyant forces acting in a quiet sea. In the case of each element of a pontoon, heave forces are out of phase with wave motion: the wave force is a positive maximum at a wave trough and a negative minimum at a wave crest. In a platform integrated from a plurality of pontoons and columns, the resultant direction of the heave force on the platform varies as a function of wave period and platform geometry, including horizontal dimension. For example, a pontoon aligned in the direction of travel of a wave and with the crest of the wave over the middle of the pontoon will experience a negative force under the crest and progressively more and more positive forces towards the troughs. Depending on the length of the pontoon and the wave period, the net force on the pontoon can either be up or down. In the case of a plurality of columns of a structure, if the columns at a given time are in troughs, the wave force on them is negative, and, conversely, if the columns are in a crest, the wave force on them is positive.
The magnitude of wave heave forces increases with wave amplitude. The longer the wave period, the deeper the wave heave forces will be felt. Large amplitude waves generally have long periods, that is not to say that large amplitude waves do not exist with shorter periods and vice versa.
What has just been described is the magnitude and direction of the dynamic forces that produce heave. These forces act on the mass of a platform and a mass of water moved by the platform in heave. This mass of water is called added mass, and its magnitude is a function only of configuration of the object that moves the water. In a right cylinder moving transverse to its generating axis, the added mass is equal to the mass of the volume of water displaced by the cylinder. Displaced mass is simply the volume of the object multiplied by the density of water: the mass of water that would occupy the space occupied by the object if the object were not there. The effective mass of an entire platform is its actual mass plus the added water mass. The actual mass includes the liks of any ballast and things carried by the platform. In a sea without waves, the actual mass of the platform is equal to its displaced mass because the platform is in equilibrium. Knowing the mass of the platform and the added mass of water is not enough to determine the platform's motion response because the third variable, force, in Newton's Second Law equation is not known. The force acting on a completely submerged object, such as a pontoon, comes out to be the sum of the mass of the water displaced by the object and the added mass of water produced by the object multiplied by the acceleration of the water without the object. The force acting on the columns can be determined by knowledge of the columns' horizontal cross-sectional area and pressure on the area as a function of time. Knowledge, then, of the displaced mass of the pontoons, the added mass of the pontoons, the cross-sectional area of the columns, the mass of the platform, the draft of the pontoons and columns, and the sea period can be used to determine the platform's motion response.
My U.S. Pat. No. 4,112,864 describes wave induced heave of a semi-submersible and a means for reducing heave by taking advantage of different phases of the dynamic forces of the waves acting on different parts of the pontoons. The patent describes a characteristic heave response of semi-submersibles. At a period of, say, 20 seconds the semi-submersible becomes resonant. Below this period there is a secondary maximum at, say, about 13 seconds at which heave response can be serious.
A tension leg platform is another type of exploration or production platform. A tension leg platform has columns extending from a deck through the water surface and below to pontoons. As before, the columns below the water surface and pontoons provide positive buoyancy. Tension legs, however, tie the platform to the ocean bottom. This restraint, obviously, can reduce heave motion by preventing the platform from moving very much in response to wave forces.
In most hostile open ocean areas the semi-submersible platform is very satisfactory for exploration and production of petroleum. However, in areas where very long waves occasionally occur, with periods as long as 24-25 seconds, the semi-submersible platform can experience serious problems because of its resonant period. In principle, it is possible to modify the design of the semi-submersible so that the natural period of heave becomes longer than 24-25 seconds to at least ameliorate the heave motion problem occurring in long wave periods. This can be done by increasing the size of the submerged pontoons, for example. If this is done, heave motion becomes worse at the shorter wave periods that are more frequently experienced. Changing the natural period can also be done by increasing the height of the columns to increase draft of the platform. But the required increase is considerable if the natural period is to be sufficiently increased to avoid the problem; the draft of the columns could be in excess of 100 meters, and the design would be expensive. In addition, with such a large draft, the semi-submersible would not be stable in the early part of its submersion with the pontoons just a short distance below the water line unless the columns canted outward, and that can lead to structural design problems.
The tension legs of tension leg platforms must be kept in tension at all times to avoid shock loads. Consequently, the legs are pre-stressed in tension a sufficient amount to accommodate expected changes in vertical forces of the platform and still be under tension. Because of considerable variations in stress, the legs are large in cross section and expensive. These variations are to a large extent due to wave forces, but there are other causes. One example is the variation in stress that occurs with wind movement of the platform. A platform moves with the wind until the horizontal component of tension balances the wind force. The horizontal movement is a component of movement in a circular arc and is therefore accompanied by an increase in draft, thus increasing the tension on the legs because of an increase in buoyant force on the platform. In deep water, this tension increase is considerable. The legs must also have enough reserve tension for variations in deck load, from drill pipe, for example, and the amount of oil and drilling fluids stored anywhere on the platform. The legs must also be strong enough to accommodate expected tides, which can be on the order of one to two meters. Aside from the requirement of variations in load that force strong legs, in very deep water, in excess of 1,000 meters, it is difficult to avoid resonant motions in heave, roll and pitch excited by normally occurring waves because of the inevitably greater elasticity of the legs. In addition, legs anchored to the sea floor may require inspection, and in deep water that can be difficult or impossible. Also, the attachment of the legs to the platform and the sea floor must accommodate fairly large angular deflections; this presents design problems.